A common method of sorting food products is based on colorimetry. Food and biological samples have rather different light scattering properties than simple solids. The detection of slight variations in color, either by visual or by optical/electrical methods, can thus be used to sort the food products. One of the primary difficulties with such methods, however, is the inability to detect slight variations in color which represent different grades of the material or which represent undesired material. For example, in the case of food products such as peanuts, sorting systems which are based on colorimetry techniques often are not able to differentiate between a desired peanut and a rock or stem which may have the approximate color and size as a desired peanut.
In addition to the basic problem of differentiating between a desired peanut and a contaminant, such as a rock or stem, it is important to be able to locate and remove contaminated peanuts. One of the contaminanants which food processors fear most is aflatoxin. This pervasive material, which is produced by the Aspergillus flavus fungus when food is improperly stored, costs the food industry millions of dollars per year. Current methods for detection of aflatoxin are time consuming and cumbersome.
For the reasons discussed above, it is apparent that there is a need for an effective inspection system for determining the quality of food products. In particular, there is a need for a system capable of differentiating between desired food products and contaminants which might not be apparent from a visual inspection of the material. The present invention overcomes the difficulties caused by standard color comparison techniques by providing an optical inspection system employing laser-induced fluorescence, as is discussed in greater detail below.
In order to understand the principles of operation of the present invention, it is imporant to understand the meaning of luminescence, as well as the historical e volution of the definition of luminescence. Historically, materials were said to exhibit characteristics of "luminescence" if they emitted photons after being irradiated with light having a wavelength in the range of approximately 1800 to 3700 Angstroms (ultraviolet). Prior art definitions of this phenomenom have included two categories: fluorescence and phosphorescence. A material was said to exhibit fluorescence if the luminescence ceased after termination of the irradiation. However, if the luminescence persisted after irradiation, the phenomena was termed phosphorescence.
The above-mentioned definitions evolved at a time when oboservations of the pesistence of luminescence were made with the unaided eye. The development of sophisticated instruments capable of measuring the persistance of luminescence for very short time periods, e.g., nanoseconds, as led to a more precise definition of the above-mentioned terms and has changed the definition of luminescence for some materials. For example, it is now known that many materials which have been characterized in the literature as being fluorescent emit luminescence for as long as 1000 microseconds after termination of excitation. This luminescence offers significant information regarding the physical characteristics of the illuminated material and in the present invention can be used to distinguish between desired food products and contaminants, as will be discussed in greater detail below.
It is well known that certain materials luminesce in the presence of ultraviolet or blue light and that the variations of the visible light luminescence can be used to determine certain features of the material. An example of an apparatus for using these phenomena to detect the presence of caries in human teeth is shown in U.S. Pat. Nos. 4,290,433 and 4,479,499 issued to Alfano. The luminescence in human teeth which is essential to the methods shown in these patents is dependent on the recognition of total visible luminescence. Further, the detection of the caries as shown therein relies on a visual recognition of differences in the color of the reradiated light from the teeth. While this luminescence technique is useful for detecting certain types of characterstics of materials, it is not suitable for an application such as that shown in the present invention because the technique is dependent on visual recognition of color differences in the luminescence of the material.
The invention method overcomes the shortcomings of pevious optical inspection systems because it takes advantage of complex excitation-luminescence spectra of peanuts. Thus, two objects which both reflect approximately the same spectrum can have different fluorescence characteristics which can be distinguished to differentiate between desired peanuts and contaminants.